This invention relates to orientation sensors for disk-shaped workpieces, such as semiconductor wafers, and, more particularly, to wafer orientation sensors for sensing the orientation of gallium arsenide wafers.
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. In one conventional approach, an ion beam is directed as a semiconductor wafer and is scanned in one direction. The wafer is translated perpendicular to the scanning direction to distribute the ion beam over the wafer. In another approach, multiple semiconductor wafers are mounted near the periphery of a disk. The disk is rotated about its central axis and is translated with respect to the ion beam to distribute the ion beam over the semiconductor wafers. A variety of different ion implanter architectures are known to those skilled in the art. The ion implanter typically includes an end station having automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after implantation.
The wafer handling system typically transfers wafers from a cassette or other wafer holder to a process station, such as a wafer mounting site. One requirement of the wafer handling system is to accurately position the wafer at the process station with its flat or notch having a predetermined angular orientation. The slots in the wafer holder are somewhat larger than the wafer and thus does not ensure accurate wafer positioning. Furthermore, the wafer flat or notch orientation is not controlled in the wafer holder. However, accurate positioning at the process station is necessary to ensure reliable wafer retention and to avoid wafer damage. In addition, ion implantation systems typically require a particular angular orientation of the wafer flat or notch, which is indicative of the crystal orientation of the wafer, to control channeling by implanted ions.
A wafer transfer system incorporating a wafer orienter is disclosed in U.S. Pat. No. 4,836,733, issued Jun. 6, 1989 to Hertel et al. A wafer is placed on an orienter chuck and is rotated. An orientation sensor includes a light source positioned below the edge of the wafer and a solar cell positioned above the edge of the wafer in alignment with the light source. The light beam from the source is directed perpendicular to the wafer surface. The wafer blocks a portion of the light beam from reaching the solar cell. As the wafer is rotated, the proportion of the light beam reaching the solar cell varies when the wafer is displaced from the center of rotation and when a fiducial, such as a flat or a notch, passes the solar cell. The signal output from the solar cell is therefore indicative of wafer offset and of the fiducial. Based on the signal from the orientation sensor, offset and angular orientation may be corrected. Wafer aligners are also disclosed in U.S. Pat. Nos. 5,452,521, issued Sep. 26, 1996 to Niewmierzycki; 5,238,354, issued Aug. 24, 1993 to Volovich; and 4,345,836, issued Aug. 24, 1982 to Phillips.
Prior art wafer orientation sensors provide generally satisfactory results with conventional silicon wafers. However, in some instances, the ion implanter is required to operate with wafers of different materials, such as gallium arsenide wafers. It has been determined that the conventional optical orientation sensor is unable to reliably sense the orientation of a gallium arsenide wafer.
Accordingly, there is a need for improved wafer orientation sensors which can sense the orientation of wafers having different properties, including gallium arsenide wafers.
Accordingly to a first aspect of the invention, a wafer orienter for a wafer having a light-transmission characteristic including a pass band and a stop band is provided. The wafer orienter comprises a mechanism for rotating the wafer and a wafer orientation sensor. The wafer orientation sensor includes a light source for emitting light, a light sensor for sensing the light emitted by the light source as an edge of the wafer is rotated between the light source and the sensor and for producing a sensor signal representative of the sensed light, and an optical filter positioned between the light source and the light sensor. The optical filter has a light-transmission characteristic wherein light in the pass band of the wafer is blocked.
In a preferred embodiment, the wafer is a gallium arsenide wafer and the optical filter blocks light in the pass band of the gallium arsenide wafer. Preferably, the optical filter blocks light at wavelengths greater than about 860 nanometers. The light source may comprise an incandescent lamp which emits light that includes wavelengths in the pass band of the gallium arsenide wafer.
According to another aspect of the invention, a method is provided for sensing orientation of a wafer having a light-transmission characteristic including a pass band and a stop band. The method comprises the steps of rotating the wafer, directing light emitted by a light source toward an edge of the wafer as the wafer is rotated, sensing the light emitted by the light source with a light sensor as the edge of the wafer is rotated between the light source and the light sensor, and blocking light in the pass band of the wafer from reaching the light sensor.